body {
    width: 1000px;
    display: flex;
}

.container1 {
    flex: 1;
    line-height: 50px;
    text-align: center;
    border: 1px solid #ccc;
    margin: 10px;
}

.container2 {
    flex: 1;
    line-height: 50px;
    text-align: center;
    border: 1px solid #ccc;
    margin: 10px;
}

.box1,
.box2,
.box3 {
    border: 1px solid #ccc;
    margin: 20px;
}